1. Field of the Invention
This invention relates in general to electrical cable, and in particular to an electrical cable for use with submersible pumps.
2. Description of the Prior Art
This invention concerns an electrical power cable used to power a downhole electrical motor for a submersible pump. These submersible pumps normally pump a mixture of oil and brine from wells often several thousand feet deep and often under high temperatures. The electrical cable normally consists of three stranded or solid conductors. Each stranded or solid conductor contains an insulating layer of a material that is resistant to oil and brine. Typically, in a round configuration, an elastomeric jacket is extruded over all three conductors and an outer metallic armor surrounds the jacket. For flat configuration cable, individually insulated and jacketed conductors are taped and braided prior to armoring in a flat configuration.
In wells that have a significant gas content, gas permeation of the jacket occurs by way of absorption which is accelerated by heat and pressure. Periodically, all submersible pumps must be pulled to the surface for servicing. As the pump is pulled to the surface, the pressure and temperature both rapidly decrease. If gas has permeated the jacket, the reduction in temperature and pressure traps low molecular weight gasses in the cable. The basically, non-porous impermeable jacket does not allow the gas to escape rapidly. The gas within expands under reduced pressure, causing the jacket to balloon, and rupture.
Improved cables are disclosed in U.S. Pat. Nos. 4,088,830 issued May 9, 1978 and U.S. Pat. No. 4,096,351, issued June 20, 1978, the inventors of both of which are Robert V. Wargin and Clinton A. Boyd. These patents teach the use of an insulating layer of thermosetting material that is resistant to oil and brine, but does allow some absorption of gas. The insulating material is relatively thin and allows gas to rapidly desorb when the cable is being pulled to the surface. A fiber braid surrounds each conductor, and contains the porous insulation layer to prevent rupturing of the insulation layer during depressurization. In the '351 patent, the conductors are surrounded by metallic armor, and in the '830 patent, the conductors are surrounded by a polypropylene, perforated layer, which serves as the armor. While the cables of these two patents perform successfully, the braid surrounding each conductor individually adds considerably to the cost of the cable.
In U.S. Pat. No. 4,472,598, Sept. 18, 1984, Clinton A. Boyd and Raymond L. Guzy, the braid is omitted, and the jacket surrounding the insulated conductors is perforated to allow gas to be released during depressurization.
In U.S. Pat. No. 3,909,467, Sept. 30, 1975, John A. Tatum, the jacket surrounding the insulated conductors contains randomly oriented flocked fibers of a non-thermoplastic material. These fibers allow the jacket to release gas absorbed therein upon depressurization. However, the jacket may not have sufficient hoop strength to prevent the insulating layer from rupturing during depressurization. This might particularly be a problem in flat cable where the jacket is of smaller diameter than in round cable.